This invention relates to the measuring and sensing of liquid levels in general and more particularly to an improved apparatus for measuring and detecting liquid levels.
Liquid level sensors presently in use typically comprise a metal rod coated with an insulating coating such as Teflon, forming one electrode of a capacitor with a tank wall forming the second capacitor electrode. By applying an RF signal across the two capacitor leads with an appropriate RF oscillator it is possible to detect and amplify changes in capacitance to provide an output indicative of liquid level or to provide an alarm if the liquid level exceeds a predetermined limit.
In another similar type of device an insulated wire is centrally located inside a metal pipe and the pipe and wire inserted into the tank, the pipe serving as one electrode for the capacitor and the wire as the other. Such an arrangement permits use in tanks where the tank is not made of metal which can act as a capacitor plate. For example, certain tanks are made of fiberglass. In instances where the materials stored in the tank are of a corrosive nature, the rod or wire is normally coated with Teflon.
These arrangements suffer from various disadvantages. The first mentioned arrangement using a rod, can only be used in tanks which are made of a conductive material. Otherwise, additional capacitive elements must be provided. Furthermore, the use of a solid rod causes difficulty in inserting the probe into certain types of tanks or devices where a clear vertical path is not present for insertion purposes. The same problem exists with a metal pipe containing a central wire electrode. This latter arrangement, whether the central electrode be a wire or a rod, has further problems in that when used with sticky materials a build up in the pipe occurs which results in inaccurate output indications.
Also of interest is a detecting apparatus disclosed in British patent specification No. 925,656. In particular, in the arrangement shown on FIGS. 3 and 4, two wires are encased in insulating material with a thin web therebetween. A similar arrangement for detecting leads is disclosed in applicant's prior U.S. Pat. No. 3,824,460. In that arrangement, the encased conductors lie on the floor and contain cutouts to improve their sensitivity to the change of dielectric constant which occurs when a liquid fills the gap between conductors. However, a direct implementation of the teachings contained in the aforementioned British specification and of the teachings in applicant's prior patent does not lead to a practical device. Applicant in attempting to come up with an improved liquid level sensor attempted to employ wires coated in a plastic material such as PVC. In testing such a device it is discovered that the results were not repeatable. Furthermore, problems were encountered in achieving adequate resolution and in using the device in tanks containing corrosives and tanks having a large degree of turbulence. It thus became evident that an improved device of this nature was needed.